Irish dancing has been around for hundreds or possibly thousands of years, and there are many different dances and steps that are constantly being renewed and changed, so to answer your question nobody will probably ever know, and it was probably not just one person, as everyone puts their own little twist on what dances are available now.
If what you are asking is What is the word for dance in Irish? It's damhsa [dhowsa] or rince [rink-ye].A dance as a function is damhsa, céilí [kaelee], or rince. For a short history on Irish dance see the link History of Irish Dance.The name for Irish dancing in Irish (Gaelic) is: rince Gaelach.
Irish dancing began with the Druids who danced during their religious rituals. This dance was then combined with Celtic dance styles as they migrated to the area.
Jig or Irish step dance
Same reason any dance is - people enjoy it. Irish dance is a very beautiful dance.
I believe it is the Single Jig/Light Jig in Irish Dancing that was once a Irish Peasant Dance.
McConoghmy School of Irish Dance
a lively Irish dance with three letters is tap dance a jig?
they dance at feises
i call clumsy Irish dance something you need to work on
Irish Modern Dance Theatre was created in 1991.
When you do an Irish dance dahhh
"Evolution" by Kin'sha